A list of the references quoted in this section is given at the end of the Description.
This invention is related to the production of high pressure air to be used for breathing purposes (medical, diving, etc) or as power source or power transmission for various air-powered tools and industrial processes. It is also related to the use of compressed air as energy storage media like in the case of compressed-air-powered cars or for example to circumvent the intermittency of some renewable energy sources such as solar and wind sources.
The potential energy of compressed air is generally exploited by firstly converting it into mechanical work. Two main categories of energy conversion systems have been proposed for that purpose: pure pneumatic conversion systems where the only active fluid is air and hydro-pneumatic conversion systems that use at least one liquid (oil, water) as active fluid.
Pneumatic Conversion Systems
Pneumatic conversion was the first (and still is the only commercially available) conversion solution used to exploit compressed air for the purpose of energy conversion. It consists, for low and medium power ranges or high compression ratios, in using mainly positive displacement (or volumetric) air machines to produce compressed air and later withdraw energy from it. In these machines, the variations of the working fluid's volume in a work-chamber produce equivalent displacements of the mechanical member, transmitting thus the energy and vice versa. The dynamic effect of the fluid is therefore of minor importance, unlike in kinetic (or turbo) machines where the kinetic energy of the working fluid is transformed into mechanical motion and vice versa. There are two main families of volumetric machines:                Rotary machines like lobe, vane, and screw machines.        Reciprocating machines like diaphragm and piston machines. In most high power and pressure ratio ranges, the piston type is commonly used because of its higher efficiency and pressure ratio.        
Since it is difficult to realise an isothermal process in these work-chambers, the compression/expansion process is subdivided into several stages and heat exchangers are inserted in between. Thus the complete cycle is more or less close to an isothermal cycle depending on the performances of the heat exchangers. This principle is as old as the first application of compressed air in propulsion in the 1800s and it is gaining nowadays more interest and improvements with the new developments in compressed-air-powered cars [1]. However, given the difficulties to implement a good heat exchange in the compression/expansion chambers, and the important leakage and friction related to the gaseous nature of air, the pressure ratings and conversion efficiency of this conversion system remain low and make it inefficient for most energy applications.
Hydro-Pneumatic Conversion
The use of hydraulic machines to circumvent the drawbacks of pure air machines has been investigated, as they suffer less from the above problems and therefore exhibit very high conversion efficiencies. One of the main challenges in using hydraulic systems to compress/expand air is the liquid-to-air interface.
A first solution has been proposed by Cyphelly & al. and is described in [2], [3] under the acronym “BOP: Batteries with Oil-hydraulics and Pneumatics”. In this system air is compressed/expanded in alternating Liquid-Piston Work-chambers where a “Thin Plate Heat Exchanger” is integrated. During compression, the thin plates transfer the heat from gas top part to the liquid bottom part and the other way round during expansion. However, good heat exchange will require a high density of plate, which is not easy to realise.
Recently, another solution has been patented by Rufer & al. with as main original proposition work-chambers where the compression is performed by injecting the liquid in the form of a “shower” in the chamber, allowing a fast and effective abortion of the compression heat [4]. This solution however requires an external liquid circulating pump to reheat the air during expansion.
In both cases, there is a concern about diffusion of the air into the liquid due to the direct contact between the two fluids. In addition, these hydro-pneumatic systems are somewhat bulky as they are assemblages of several distinct components and machines. Moreover, these split topologies require many ancillary devices for the command and control of the system's operation.
U.S. Pat. No. 1,929,350 discloses an apparatus for compressing gases which has an external heat exchanger circulating external cooling liquid through tubes passing through the expansion/compression chamber.
It would be desirable to provide a hydro-pneumatic conversion system with a simple and efficient integrated heat exchanger that can effectively operate both during compression and expansion. It would be also desirable to have a more compact, flexible and scalable solution that can be easily adapted to stationary as well as mobile applications. The present invention proposes original solutions to achieve these objectives.